Conventional radio-controlled clocks are designed to receive a radio wave amplitude-modulated by time information serving as an official time and frequency standard for a corresponding country; this radio wave is continuously broadcasted from a transmitter located in a time zone where the clocks are used.
The conventional radio-controlled clocks are also designed to demodulate the time information, and correct a measured time based on the demodulated time information. An example of the conventional radio-controlled clock is disclosed in Japanese Unexamined Patent Publication No. 2003-60520.
Specifically, a radio-controlled clock includes an internal oscillator, such as an internal quartz oscillator, and a measuring circuit operative to measure a time based on the oscillator frequency (reference frequency), receive the time information on the radio wave broadcasted from a transmitter, and periodically correct a measured time based on the time information.
If the oscillator frequency (reference frequency) contains a frequency error with respect to the correct frequency, an error contained in a measured time would increase over time. For this reason, a radio-controlled clock is designed to periodically correct a measured time based on the received time information serving as an official time and frequency standard, thereby keeping a measured time to the right time.
Typically, the quartz oscillator frequency is within a few parts per million of the correct frequency, which means it can keep time to within 1 second for a few days or more. Therefore, the correction once per day or a few days make the clock appear to be on the right second.
The time information consists essentially of a time code with a predetermined frame format. The time code includes 60 bits of information, broadcasted at 1 bit per second. Specifically, the time code frame is broadcasted from a transmitter every 60 seconds (1 minute).
For example, in Japan, the time code bits are generated by riding and lowering the power (amplitude) of a carrier wave whose frequency is 60 kHz or 40 kHz with a modulation range of 90% (the ratio of the high amplitude and low amplitude is 10:1.
If there is bad radio reception (low S/N ratio in radio waves), the time information is not always demodulated accurately. For this reason, in order to prevent a measured time from being corrected based on inaccurately demodulated time information, verification of demodulated time information is carried out, which is disclosed for example in Japanese Unexamined Patent Publication NO. 2001-108770.
Specifically, as the first check means, for each of the demodulated time code frames, it is checked whether the time code bits meet a predetermined time code format for each time code frame. When it is checked that the time code bits in a demodulated time code frame do not meet the predetermined time code format, it is determined that an error arises in the demodulated time code frame.
As the second check means, a parity is computed for at least part of the time code bits in a current demodulated time code frame, and that is computed for at least part of the time code bits in the next current demodulated time code frame.
Then, it is checked whether the parity of the current demodulated time code is matched with that of the next demodulated time code frame.
When it is checked that the parity of the current demodulated time code frame is mismatched with that of the next demodulated time code frame, it is determined that an error arises in at least one of the current and the next demodulated time code frames.
As the third check means, some bits allocated for “minutes” between two of demodulated continuous time code frames are compared with each other to check whether increment of some bits allocated for “minutes” between two of the demodulated continuous time code frames corresponds to one minute.
When it is checked that increment of some bits allocated for “minutes” between two of the demodulated continuous time code frames does not correspond to one minute, it is determined that an error arises in at least one of the two of the demodulated continuous time code frames.
If it is determined that an error arises in a demodulated time code frame based on any one of the first to third check means, correction of a measured time based on the error-detected time code frame is prevented.
As described above, the second and third check means need to receive at least two continuous time code frames. This takes a few minutes and more to correct a measured time because a time code frame is broadcasted from a transmitter every 60 seconds (1 minute). This may increase power consumption of processes except for the normal time measuring process based on the reference frequency.
Especially, in battery-powered radio-controlled clocks, the power-consumption increase may decrease a life of the battery.
In addition, in order to execute the second and third check means, at least two continuous time code frames need to be properly received by a radio-controlled clock. For this reason, when a radio-controlled clock is put in a place where there is bad radio reception, correction of a measured time may have not been carried out for a long period of time. This may make it difficult for the radio-controlled clock to properly measure time.